Can some explain this strange behavior of the hypergeometric distribution in scipy?

Question

I am running Python 2.6.5 on Mac OS X 10.6.4 (this is not the native version, I installed it myself) with Scipy 0.8.0. If I do the following:

>>> from scipy.stats import hypergeom
>>> hypergeom.sf(5,10,2,5)

I get an IndexError. Then I do:

>>> hypergeom.sf(2,10,2,2)
-4.44....

I suspect the negative value is due to bad floating point precision. Then I do the first one again:

>>> hypergeom.sf(5,10,2,5)
0.0

Now it works! Can someone explain this? Are you seeing this behavior too?

Answer 1

I don't know python, but the function is defined like this: hypergeom.sf(x,M,n,N,loc=0)

M is the number of interesting objects, N the total number of objects, and n is how often you "pick one" (Sorry, German statistician).

If you had a bowl with 20 balls, 7 of those yellow (an interesting yellow), then N is 20 and M is 7.

Perhaps the function behaves undefined for the (nonsense) case when M>N ?